The development of new materials and processes for the production of metal pieces with the aim of reducing component weight at a low cost is of utmost importance in the automotive industry. In order to achieve these objectives, the industry has developed ultra-high-strength steels (UHSS) which exhibit an optimized maximal strength per weight unit and advantageous formability properties. These steels are designed to attain a microstructure after heat treatment, which confers good mechanical properties and makes them especially suited for the hot stamping process used to form steel blanks into particular automobile parts. Since during the hot stamping process the blank is subjected to aggressive atmospheres, the steel is usually coated to avoid corrosion and oxidation.
In an attempt to minimize the weight of components while respecting structural requirements, so-called “tailored blank” techniques may be used. In these techniques, components may be made of a composite metal blank which is obtained by welding several blanks with different thicknesses, size and properties. At least theoretically, using this kind of technique the use of material may be optimized. Blanks of different thickness may be joined or a steel blank may be joined with a coated steel blank for example, using the specific properties of each material where they are needed.
These blanks may be welded “edge to edge” (“butt-joining”). These so-called tailored blanks, are designed to be hot stamped and afterwards be manufactured to form automotive parts. Tailored welded blanks may be used for structural components such as doors, B-Pillars, beams, floor, etc.
Similarly “patchwork” blanks are known, in which several blanks are not necessarily welded “edge-to-edge”, but instead partial or complete overlaps of blanks may be used.
An example of steel used in the automotive industry is 22MnB5 steel. In order to avoid the decarburization and the scale formation during the forming process, 22MnB5 is presented with an aluminum-silicon coating. Usibor® 1500P and Ductibor® 500P, commercially available from Arcelor Mittal, are examples of steels used in tailored and patchwork blanks.
Patchwork blanks and tailored blanks may also be used or useful in other industries.
Usibor® 1500P is supplied in ferritic-perlitic phase. It is a fine grain structure distributed in a homogenous pattern. The mechanical properties are related to this structure. After heating, a hot stamping process, and subsequent quenching, a martensite microstructure is created. As a result, maximal strength and yield strength increase noticeably.
The composition of Usibor is summarized below in weight percentages (rest is iron (Fe) and unavoidable impurities):
CSiMnPSCrTiBN0.240.271.140.0150.0010.170.0360.0030.004
As mentioned before, Usibor 1500 is supplied with an aluminum-silicon (AlSi) coating in order to prevent corrosion and oxidation damage. However, this coating has a significant downside related to its weld behavior. If Usibor blanks are welded without any further measures, aluminum of the coating may enter into the weld area and this can cause an important reduction of the mechanical properties of the resulting component and increase the possibility of weak fracture in the weld zone.
In order to overcome this problem a method was proposed in DE202007018832 U1 which consists in removing (e.g. by laser ablation) a part of the coating in an area close to the welding gap. This method has the disadvantage that an additional step is needed for the production of the (tailored) blanks and components and that in spite of the repetitive nature of the process this additional step requires a complex quality process with an elevated number of parts which are to be scrapped. This entails an increase of the cost of the welding step and limits the competitiveness of the technology in the industry.
WO 2013/045497 discloses a method for butt welding coated sheet metals, wherein at least one powder welding additive in the form of a gas-powder flow is fed to the welding melt with a speed of 2 m/s-50 m/s. A relatively high speed is needed in order to achieve mixing of the powder additive in the weld zone.
The problems of poor weldability due to aluminum entering in the weld zone are not only known from welding plates or blanks of coated steels, but may also be found e.g. when welding two aluminum blanks or when welding a steel blank (with or without coating) with an aluminum blank.
Herein a blank may be regarded as an article which has yet to undergo one or more processing steps (e.g. deformation, machining, surface treatment or other). These articles may be substantially flat plates or have more complicated shapes.
In examples of the welding methods described herein the aforementioned disadvantages are avoided or at least partially reduced.